1. Field of the Invention
The invention relates generally to methods and apparatus for generating and detecting stress waves in members, and more particularly in members composed of piezoelectric materials.
2. Description of the Prior Art
Certain structural properties, such as mechanical integrity, of members have been studied by ultrasonic techniques. An ultrasonic signal applied to a member generates a stress wave in a structure. Certain characteristics of the detected stress waves, such as relative amplitude, phase, transmission co-efficient, and so forth, may indicate characteristics of the member. Stress wave techniques, which can be non-invasive, are often preferable to invasive techniques, as they avoid destruction of the member under study.
Stress wave techniques are particularly useful in studying certain mechanical properties of bones. Analysis of stress waves can be used to indicate, for example, the amount of healing of a bone fracture, and can also be used to detect the onset and progression of diseases such as osteoporosis long before they can be sensed by standard radiographical techniques.
In studying bone structures, a major drawback to the use of stress wave or vibration techniques is the soft tissue surrounding the bone. Prior art techniques include impedance methods and vibration methods in which the response was measured by pressing accelerometers on the skin above the bone. However, the characteristics of these measurements were significantly affected by the thickness and quality of soft tissue between the accelerometer and bone, as well as by the amount of preload force with which the accelerometer was pressed on the soft tissue. This suggests that in these methods the variations in the quantity and quality of soft tissue from patient to patient constitute a complicating variable which cannot be easily evaluated. It is therefore desirable to provide a method and apparatus for generating stress waves in the bone and for detecting the stress waves which is independent of the mechanical properties of the soft tissue. Prior art techniques included attempts to vibrate the whole bone, which yielded inconclusive results. Other prior art techniques involved using a pin, such as traction pins used in management of some fractures, embedded in the bone at the detector location. The pin vibrates in response to stress waves induced in the bone at that location. This vibration would be independent of the soft tissue effects; however, this approach can be used only for patients having traction pins in the bone. The use of traction pins is an invasive technique, and non-invasive techniques are strongly preferred in this area.
In some materials, such as bone, when the material is mechanically stressed, a magnetic field is generated in response to, and proportional to, the applied stress. An applied stress is propagated as a stress wave through a member in a manner similar to a sound wave propagating therethrough, and, as the stress wave is propagated, the magnetic field follows.
The present invention provides a method and apparatus for studying structural properties of members by providing method and apparatus for generating and detecting stress waves in the members. The method and apparatus facilitate a non-invasive technique for studying certain properties of members such as bones by stress wave techniques which avoids the effects of the soft tissues.